The First Stars and Galaxies
Understanding the formation and properties of the first stars and galaxies is one of the prime objectives in modern cosmology. Advances in computational power and simulation software, e.g. the introduction of hydrodynamic AMR and SPH solvers, have made it possible to directly model the growth of structures in the Universe, leading to the formation of the first stars and galaxies. This revolution has brought a great deal of theoretical insight, and we anticipate that future probes such as the James Webb Space Telescope (JWST) will confirm the current model.
The First Stars
The formation of the first stars, termed "Population III" or simply "Pop III", marks a profound transition in the early Universe. With the emission of the Cosmic Microwave Background (CMB), the Universe remains dark for approximately 400 million years. During this period, primordial quantum fluctuations grow and subsequently become large enough to initiate the collapse of the first bound structures. These primordial "minihalos" are the formation sites of the first stars, as molecular hydrogen allows the gas to cool and form protostellar seeds. Although uncertainties concerning the influence of magnetic fields and radiative feedback during the accretion process remain, numerical simulations indicate that these stars grow to be typically 100 times more massive than the Sun (e.g. Bromm & Larson 2004).
Radiative Feedback and Subsequent Stellar Generations
The high masses and surface temperatures of typical Pop III stars give rise to enormous photon yields, which ionize the intergalactic medium at distances up to 5 kpc around the star (e.g. Alvarez, Bromm & Shapiro 2006). Furthermore, radiation in the so-called Lyman-Werner bands dissociates hydrogen molecules in the surrounding gas and prevents efficient cooling (e.g. Machacek, Bryan & Abel 2001). In a recent study, we have investigated both types of feedback and shown that Pop III star formation is suppressed by a factor of a few, primarily due to potoheating (Johnson, Greif & Bromm 2007). However, once these relic H II regions recollapse, efficient formation of hydrogen deuteride (HD) sets in and enables the gas to cool to the level of the CMB. This sets a characteristic stellar mass scale that is one order of magnitude lower than Pop III. With respect to their intermediate status regarding formation environemt and typical masses, these stars have been termed Pop II.5 (Mackey, Bromm & Hernquist 2003, Johnson & Bromm 2006, Greif & Bromm 2006).
Depending on their masses, Pop III stars experience very different fates after their main-sequence lifetimes of 2-3 million years. On the lower end, the star collapses to a black hole without any significant expulsion of metals, whereas on the higher end a so-called pair-instability supernova (PISN) disrupts the entire star in an extremely violent explosion (e.g. Heger et al. 2003). The latter scenario leads to an efficient dispersion of metals into the intergalactic medium, and paves the way for the formation of modern stellar populations (e.g. Bromm et al. 2001). Current work concentrates on simulating the explosion of a 200 solar mass PISN in the early Universe after the formation of the first star, and deriving the expansion and chemical enrichment properties (see here).